PROJECT SUMMARY During bacterial protein synthesis, the 30S and 50S ribosomal subunits assemble into the translationally active 70S ribosome on template mRNA. In the Gram-positive human bacterial pathogen Staphylococcus aureus, a single small ribosome-binding protein called hibernation-promoting factor (HPFSa) stimulates the dimerization of 2.5-MDa 70S monomers to form the translationally silent 100S complex. The physiological function of the 100S ribosome remains enigmatic because the temporal abundance of the 100S ribosome varies considerably among different bacterial phyla, the global impact of the 100S ribosome on translation is completely unknown, and hpf null mutants of different bacteria lack a common phenotype. Moreover, distantly related gammaproteobacteria, such as E. coli, require two proteins (RMFEc and HPFEc) to achieve 100S complex formation. Recent data from our group demonstrate that HPFSa is essential for the bacterial survival and maintenance of the ribosome pool in aging S. aureus cells. Surprisingly, eliminating hpfSa causes the derepression of only a subset of genes at translational initiation. Our goal is to establish a mechanistic understanding of the function of the 100S ribosome in translational capacity and staphylococcal pathogenesis. We will take a multi- disciplinary approach that spans genetics, molecular biophysics, biochemistry, and whole animal infection studies. Aim 1 will determine the process and factors involved in the reversible conversion of 70S and 100S ribosomes. Aim 2 will determine how the HPFSa/100S ribosome inhibits translation in a gene-specific manner. Aim 3 will identify the roles of the 100S complex in ribosome turnover and staphylococcal pathophysiology. These aims have the potential to produce novel insights into ribosome metabolism and inspire alternate treatments for persistent and relapsed staphylococcal infections that are intimately linked to survive for an extended period inside the host.